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BIO DESIGN
Mini Review P 135-141
Role of CRTC2 in the control of hepatic
gluconeogenesis
Hye-Sook Han1 and Seung-Hoi Koo1*
1
Division of Life Sciences, College of Life Sciences & Biotechnology, Korea University, Seoul 136-713, Korea.
*Correspondence: [email protected]
Fasting glucose metabolism in the liver is critical in maintaining energy homeostasis in mammals. To provide sufficient
amounts of glucose in the bloodstream, a preferred fuel for most tissues, glucose production from the liver is activated
under starvation conditions. Short-term fasting elicits the production of glucose from glycogen, a storage form of glucose
in the liver, by activating glycogenolysis. Longer-term fasting or starvation also triggers the activation of de novo glucose
synthesis, or gluconeogenesis, by utilizing various precursors such as lactate, amino acids, or glycerol. Activation of
the latter pathway is achieved mainly via transcriptional mechanism, and fasting hormone glucagon or stress hormone
epinephrine induces cAMP-dependent pathway in the liver, thus activating transcription factor cAMP response element
binding protein (CREB) and the resultant transcription of downstream target genes in the gluconeogenesis including
phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase catalytic subunit (G6Pase). While CREBdependent transcriptional pathway is activated under fasting conditions, feeding effectively reduces glucose production
from the liver in part by reducing CREB-dependent transcriptional activation of gluconeogenic gene expression.
Increased hepatic glucose production under insulin resistance or type 2 diabetes is an important underlying mechanism
for hyperglycemia, and it was shown that hyperactivation of CREB-dependent transcription can be attributable for such
phenomenon. In this review, we would like to delineate the mechanistic insight into the role of CREB regulated transcription
coactivator 2 (CRTC2), a new transcriptional coactivator of CREB, in the control of hepatic gluconeogenesis.
INTRODUCTION
The liver is a major organ to regulate energy homeostasis
in mammals. It plays a central role in the control of glucose
metabolism, lipid metabolism, as well as detoxification
processes that is essential for the everyday life. In particular,
it plays a key role in the metabolism of glucose, a key nutrient
that is mainly utilized by most species from bacteria to men.
Under feeding, dietary carbohydrate is processed in the intestine
and the resultant monosaccharide, which is mostly glucose, is
transported into the various tissues via bloodstream (Pilkis and
Claus, 1991; Pilkis et al., 1988). The glucose in excess is stored in
the tissues as a storage form of glucose, glycogen. In particular,
the liver stores large amount of glycogen that can be utilized by
tissues that are mainly dependent on the glucose as a preferred
fuel such as brain and red blood cells. Under fasting conditions,
the stored glycogen is mobilized as glucose via a process of
glycogen breakdown or glycogenolysis to provide glucose to the
various tissues. Under longer term fasting or in time of starvation,
the liver provides more glucose by activation of de novo glucose
synthesis or gluconeogenesis. The precursor of gluconeogenesis
includes lactate, amino acids and glycerol. Lactate is derived
from the anaerobic glycolysis mainly from the skeletal muscle
and is transport to the liver via Cori cycle. Starvation process
also triggers protein breakdown and provides its building block
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amino acids as precursors for the gluconeogenesis. Fasting also
triggers lipolysis in the adipose tissues, resulting in the increased
serum free fatty acids as well as glycerol. While free fatty acids
can be taken up by various tissues and are utilized as alternative
fuels, glycerol taken up by the liver is converted into glycerol
3-phosphate and is introduced into the gluconeogenesis.
Glucagon is a major hormone that is responsible for the
mobilization of glucose under fasting conditions. Low blood
glucose triggers increased secretion of glucagon from the
pancreatic alpha cells, which in turn activates intracellular
cAMP signaling pathway by activation of G protein signaling
and adenylate cyclase activity mostly in the liver. cAMP directly
activates protein kinase A (PKA), which is responsible for the
increased glycogenolysis as well as hepatic gluconeogenesis
by increased phosphorylatio n of enzymes in the pathway.
In addition, PKA is also involved in the chronic activation of
hepatic gluconeogenesis by enhancing transcription of key
gluconeogenic genes including PEPCK, G6Pase, and pyruvate
carboxylase (PC). cAMP response element binding protein
(CREB) is responsible for such transcriptional regulation in
response to the activation of cAMP pathway (Herzig et al., 2001;
Moore et al., 1998). Briefly, cAMP binds to the regulatory subunits
of PKA, releasing its catalytic subunits. Activated PKA enters
the nucleus and phosphorylates serine 133 residue of CREB
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135
Role of CRTC2 in the control of hepatic gluconeogenesis
that is bound to the cAMP response element (CRE) on the target
promoter. Phosphorylated CREB undergoes a conformational
change that enables it to recruit CREB coactivators CREB
binding protein (CBP)/p300, thus activating target gene
transcription. Indeed, CRE sequences have been identified in the
5’-upstream sequences of various gluconeogenic genes such as
PEPCK, G6Pase, PC, and fructose 1,6-bisphosphase (Fbpase1).
Subsequently, two other mammalian paralogues, CRTC2 and
CRTC3 were identified in mammals. Indeed, CRTC orthologues
were also identified in lower eukaryotes including Caenorhabditis
elegans and Drosophila Melanogaster, showing a conservation
of this CREB coactivator family during evolution (Altarejos and
Montminy, 2011).
In addition, peroxisome proliferator activated receptor gamma
coactivator 1 alpha (PGC-1α) also contains CRE on its promoter
sequence and is transcriptionally induced by CREB. Since PGC1α is an important coactivator for fasting glucose and lipid
metabolism in the liver, its transcriptional regulation indicates the
importance of cAMP-dependent transcriptional control in energy
homeostasis in the liver. The importance of CREB in the control
of hepatic gluconeogenesis has been verified in in vivo settings
using animal models. Transgenic mice expressing A-CREB,
a CREB inhibitor, in the liver reduces gluconeogenic gene
expression in the liver (Herzig et al., 2001). Adenovirus mediated
expression of A-CREB in the liver reduces gluconeogenic gene
expression including PEPCK, G6Pase, PC, and PGC-1α, with a
concomitant reduction in blood glucose levels. In addition, acute
delivery of shRNA against CREB in the liver leads to the reduced
hepatic gluconeogenesis, confirming the importance of CREB in
the control of hepatic gluconeogenesis in vivo (Erion et al., 2009).
Recently, a new class of CREB coactivator, CREB regulated
transcriptional coactivators (CRTCs), has been characterized
(Conkright et al., 2003). Among the family members, CRTC2 is
most abundant in the liver, and is linked to the CREB-dependent
control of hepatic glucose metabolism (Koo et al., 2005). In this
review, we would like to delineate the recent findings regarding
the role of CRTC2 in the glucose metabolism in the liver as well
as molecular mechanisms in controlling its activity in detail.
The structure of CRTC2 and its posttranslational modifications
Identification of CRTC proteins
CRTC proteins consist of an amino terminal domain termed
CREB binding domain that is responsible for the binding of
CRTC proteins with CREB and other basic leucine zipper
(bZIP) transcription factors, a carboxy terminal domain termed
transactivation domain, and a central regulatory serine/proline
rich domain that is mainly targeted by various post-translational
modifying enzymes for regulating activity of this protein family
(Figure 1) (Altarejos and Montminy, 2011). Recently, the function
of amino terminal CREB binding domain was delineated in
detail (Luo et al., 2012). According to the study, 28 residues of
the amino terminus of CRTC2 consist of alpha helix structure,
and directly bind to leucine zipper region of CREB. Since alpha
helix of CRTC2 lacks basic amino acids, CRTC2 is not directly
involved in the DNA binding, but promotes stronger interaction
among CREB, CRTC2 and its cognate DNA (containing CRE
sequences) via non-specific interaction with phosphate group of
DNA.
Several reports also attempted to characterize the nature
of transactivation domain of CRTC2. It appeared that Lys628
residue of CRTC2 is targeted by a couple of post-translational
modification marks. Acetylation of Lys628 of CRTC2 by
CBP/p300 led to the activation of this protein via tighter
association of CREB-containing transcriptional complex (Liu et
al., 2008). On the other hand, histone deacetylase Sirt1 promotes
deacetylation of Lys628, leading to the dissociation of CRTC2containing transcription machinery. Lys628 is also target of COP1
CRTC proteins were first identified as a new regulator of cAMPdependent transcriptional events by expression
library screening using CRE luciferase (Conkright
et al., 2003). CRTC1, the founding member of
CRTC family of proteins, was shown to increase
CRE luciferase activity in response to cAMP
treatment in the HEK 293 cells. The CRTC1 was
first reported as a fusion gene in the name of
MECT1-MAMML2. Via a recombination event,
DNA sequences encoding the CREB binding
domain of CRTC1 (termed Mucoepidermoid
carcinoma translocated-1 (MECT1)) was fused to
Figure 1 i Post-translational modification on CRTC2. Post-translational modifications
on CRTC2 are shown. PRMT6 catalyzes asymmetric dimethylation on arginine residues
the C-terminal transcriptional activation domain
(R51, 99, 120, and 123) in the nucleus. AMPK/SIK phosphorylates, while PP4/Calcineurin
of the Notch coactivator Mastermind-like 2
dephosphorylates serine 171 residue. The same serine 171 residue is targeted by OGT
(MAML2) leading to the generation of a fusion
for O-GlcNAcylation. Acetylation of lysine 628 by CBP/p300 activates CRTC2, while
deacetylation of this residue by SirT1 inhibits it. Lys628 is also targeted by COP1 ubiquitin
gene encoding strong transcriptional activator
ligase for polyubiquitination. A filled circle indicates asymmetric dimethylation, while an open
found in the Mucoepidermoid carcinoma. As
circle indicates ubiquitination. A filled triangle is for O-GlcNAcylation, and an open triangle
a transcriptional coactivator CRTC1 does
denotes phosphorylation. Finally, a filled square indicates acetylation. CBD: CREB binding
domain; RD: Regulatory domain; TAD: transactivation domain
not possess an intrinsic DNA binding motif.
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Hye-Sook Han and Seung-Hoi Koo
ubiquitin ligase. Polyubiquitination of CRTC2 is then targeted by
proteasome and is subsequently degraded (Dentin et al., 2007).
Thus, acetylation of Lys628 protects CRTC2 from ubiquitin
proteasome mediated degradation and termination of cAMPdependent transcriptional signal.
Recruitment of other transcriptional regulator by transactivation
domain of CRTC2 was recently suggested. In the report, they
by promoting histone acetylation (Krones-Herzig et al., 2006).
However, the direct interaction between this protein and CRTC2
was not tested. Recent report suggests that PRMT5 can
directly bind to CRTC2 and also promotes histone acetylation of
gluconeogenic promoters to activate CREB-CRTC2 dependent
transcription in the liver (Tsai et al., 2013). In either case, direct
modification of CRTC2 by PRMTs was not shown. On the other
showed that the transactivation domain of CRTC2 associates
with a class of histone acetyltransferase KAT2B, leading
to the increased acetylation of histone 3 lysine 9 (H3K9) of
gluconeogenic promoters, resulting in the increased association
of CREB-CRTC2 and transcriptional activation of target genes
(Ravnskjaer et al., 2013).
Serine 171 residue in the central regulatory domain of CRTC2
is a key site for the regulation of this protein. In the absence of
cAMP signaling, AMPK and its related kinases are activated,
thus resulting in the phosphorylation of serine 171 of CRTC2
and its tight association with 14-3-3 proteins in the cytosol.
AMP activated protein kinase (AMPK) and its related kinases
(AMPKRKs), including salt inducible kinase (SIK)1 and SIK2, are
associated with the phosphorylation of this residue (Screaton
et al., 2004). Upon activation of cAMP signaling pathway under
fasting conditions, PKA phosphorylates and inactivates these
kinases, leading to dephosphorylation and activation of CRTC2.
Recent reports also suggested the role of serine/threonine
phosphatases in the control of dephosphorylation event of
CRTC2. Serine/threonine protein pho sphatase 4 (PP4) is
activated under fasting or in insulin resistant states, in part via
transcriptional activation of Suppressor of MEK null (SMEK)1 and
SMEK2, a pair of PP4 regulatory subunit 3 proteins, leading to the
increased dephosphorylation and activation of CRTC2 (Yoon et
al., 2010). CREB also induces expression of inositol 3 phosphate
receptor, resulting in the release of calcium from the ER stores
under fasting conditions. Increase calcium concentration in
turn activates calcineurin, a PP3 catalytic subunit, which also
promotes dephosphorylation and activation of CRTC2 in the
liver (Wang et al., 2012). Dephosphorylation of CRTC2 on serine
171 residue leads to the dissociation of CRTC2 from 14-3-3 and
subsequent nuclear translocation. Thus, concomitant inactivation
of kinases and activation of phosphatases result in the activation
and the nuclear entry of CRTC2 under fasting conditions, leading
to the activation of gluconeogenic gene transcription.
In addition to the phosphorylation/dephosphorylation,
serine 171 is also shown to be O-GlcNAcylated by O-linked
N-acetylglucosamine (O-GlcNAc) transferase (OGT) under
hyperglycemic conditions such as type 2 diabetes (Dentin
et al., 2008). O-GlcNAcylated serine residue is refractory to
the phosphorylation by AMPK family of kinases, providing a
potential mechanism for the hyperactivation of CRTC2 during
hyperglycemia in conditions such as type 2 diabetes.
Protein arginine methyltransferases (PRMTs) have been also
shown to be involved in the control of CRTC2 activity. PRMT4
interacts with CREB and activates CREB-dependent transcription
hand, PRMT6 promotes CRTC2 activity via a direct asymmetric
dimethylation of arginine residues of CRTC2 (Han et al., 2014).
PRMT6 resides in the nucleus, and directly methylates arginine
residues of CRTC2 in its amino terminus, thus enhancing the
interaction between CREB and CRTC2 onto the gluconeogenic
promoters. Interestingly, the identified arginine residues (arginine
51, 99, 120, and 123) are conserved among various species
as well as other paralogues (CRTC1 and CRTC3), showing the
importance of this modification in the control of CRTC2 activity.
Future study will clarify the relative contribution among these
PRMTs in the control of CRTC2-dependent transcriptional
pathway in the liver.
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Direct control of hepatic gluconeogenesis by CRTC2
CRTC2 is a predominant isoform among CRTC proteins in the
liver. Under fasting conditions, pancreatic alpha cell secretes
glucagon, leading to the activation of glucagon receptor
mediated signal transduction mainly in the liver. Activation of
PKA results in the activation of serine/threonine phosphatases
and the inactivation of serine/threonine kinases, promoting
dephosphorylation and nuclear translocation of CRTC2. In
the nucleus, CRTC2 binds to CREB on the gluconeogenic
promoters such as PEPCK, G6Pase, and PGC-1α, and turns on
gluconeogenesis (Koo et al., 2005).
The role of CRTC2 in the hepatic gluconeogenesis has been
also confirmed in in vivo animal models. Depletion of hepatic
CRTC2 by RNA interference lowered blood glucose levels with
reduced gluconeogenic gene expression (Koo et al., 2005;
Saberi et al., 2009). A couple of CRTC2 knockout mice models
were suggested. In one model, deletion of exon 4 through 11
of CRTC2 did not elicit changes in the blood glucose levels in
mice, but showed a reduced response to cAMP treatment in
hepatocytes, suggesting that CRTC2 may not function as a
regulator of glucose metabolism in vivo (Le Lay et al., 2009).
However, careful investigation reveals that the deletion of
exons 4 through 11 led to the potential generation of in-frame
mutant form of CRTC2. Indeed, deletion of exons 4 through
11 removes most of the regulatory domain of CRTC2 including
serine 171 residue, resulting in the generation of constitutively
active form of CRTC2, at least in the cultured cells (Altarejos and
Montminy, 2011). Thus, further characterization is necessary
before fully accepting the result of the in vivo study utilizing these
knockout mice. Another systemic knockout mouse of CRTC2
was generated and reported recently. In this line of mice, exon
1 of CRTC2 was targeted for the generation of knockout mice,
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Role of CRTC2 in the control of hepatic gluconeogenesis
table 1 i Metabolic phenotypes of CRTC knockout animals
Isoform
CRTC1
Site of Major Expression
Brain (Hypothalamus)
Major Target Genes
Knockout phenotype
CART1, IRS2
Hyperphagia, Obesity, Infertility
CRTC2
Liver
PEPCK, G6Pase, PGC-1α, ERRγ, Lipin1
Reduced hepatic
gluconeogeneis/
Improved insulin
sensitivity
CRTC3
Adipocytes
RGS2
Resistant to diet-induced obesity,
Improved insulin sensitivity
dTORC
(D. Melanogaster)
Brain
Cox, catalase
Reduced resistance against
oxidative stress, Reduced lifespan
CRTC-1
(C. Elagans)
Ubiquitous
Abu (ER stress related gene)
Increase lifespan
precluding the potential generation of chimeric form of CRTC2
that could hinder the interpretation of the result that is associated
with CRTC2 KO mice (Wang et al., 2010). Indeed, CRTC2 KO
mice displayed lower blood glucose levels, and improved
glucose tolerance with reduced gluconeogenic gene expression
in the liver, confirming the critical role of CRTC2 in the control of
hepatic gluconeogenesis in vivo.
Modulation of target genes of CRTC2:
indirect regulation of glucose
metabolism by CRTC2
As a transcriptional coactivator of CREB, CRTC2 regulates
hepatic glucose metabolism not only via a direct transcriptional
control of gluconeogenic genes, but also via an indirect
mechanism by controlling transcription of genes that encode
various proteins affecting glucose homeostasis (Table 1).
As described previously, CREB and CRTC2 are involved in
the transcriptional activation of PGC-1α. First as identified as a
coactivator for peroxisome proliferator activated protein (PPAR)
γ in brown adipocytes, PGC-1α functions as a coactivator for
various nuclear hormone receptors including PPARα, PPARδ,
liver X receptors (LXRs) and glucocorticoid receptor (GR) via a
direct interaction by using LXXLL motifs (Finck and Kelly, 2006;
Puigserver et al., 1998). In the liver, PGC-1α is shown to promote
gluconeogenic gene expression by coactivating hepatic nuclear
factor 4 (HNF4) α and FoxO1 (Puigserver and Spiegelman,
2003). While depletion of CRTC2 by RNA interference reduced
gluconeogenic gene expression, the effect was effectively
rescued by ectopic expression of PGC-1α, showing indeed
that CREB and CRTC2 are involved in the transcriptional
control of hepatic gluconeogenesis both via a direct activation
of gluconeogenic genes and an indirect mechanism through
activation of PGC-1α (Koo et al., 2005).
Estrogen receptor related (ERR) γ has also been shown to a
direct transcriptional target of CREB and CRTC2 in the liver
(Kim et al., 2012). As in the case of PGC-1α, expression of ERR
γ is transcriptionally activated by CREB and CRTC2-dependent
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manner under fasting conditions. Inhibition of ERR γ activity by
either RNA interference or inverse agonist GSK5182 restores
euglycemia in a mouse model of type 2 diabetes. Furthermore,
ERR γ also controls gluconeogenic gene expression in
conjunction with PGC-1α, further showing the case for the
control of hepatic gluconeogenesis by CREB and CRTC2 via an
indirect pathway.
Lipin1 is another transcriptional target of CREB and CRTC2
whose expression is increased under fasting or by insulin
resistance in the liver (Ryu et al., 2009). Lipin1 encodes a protein
termed LPIN1, a phosphatidic acid phosphatase that catalyzes
the conversion of phosphatidic acids to diacylglycerol (DAG) on
the surface of smooth endoplasmic reticulum (ER). Increased
expression of lipin1 under obesity results in the accumulation
of DAG. DAG-mediated signaling pathway aggravates insulin
resistance and enhances hepatic glucose production, which
contributes to the hyperglycemic phenotype. Thus, CRTC2dependent transcription mechanism is shown to be a novel
target of controlling insulin resistance and hyperglycemia. Based
on this result, further study will be necessary to delineate other
potential CREB-CRTC2 target genes that could impact on the
control of hepatic glucose metabolism. Utilization of CRTC2
knockout mice will be a valuable asset to conduct such a study
in the future.
CRTC2 as a transcriptional coactivator
for other bZIP family of proteins
Since CRTC2 functions as a transcriptional coactivator of
CREB by its interaction with bZIP domain, it is plausible to
expect that CRTC2 could interact with other members of bZIP
factors. Indeed, activating transcription factor (ATF) 6, a class
of ER-bound bZIP transcription factor, was shown to interact
with CRTC2 in the liver (Wang et al., 2009). ATF6 is a critical
transcriptional activator in response to ER stress, and enhances
expression of key gene products in the unfolded protein
response (UPR) pathway. Normally resides in the ER, ATF6 is
activated under ER stress by proteolytic cleavage that would
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Hye-Sook Han and Seung-Hoi Koo
release an active transcription factor moiety into the nucleus.
According to the recent report, ER stress also triggers the ERstored calcium release in the liver, leading to the activation of
calcineurin and resultant dephosphorylation/activation of CRTC2.
Thus, concomitant activation of ATF6 and CRTC2 enable them to
function as an active transcriptional machinery to activate genes
involved in the UPR pathway.
Another ER-bound bZIP factor CREBH was also shown to
interact with CRTC2, using it as a transcriptional coactivator in
the liver (Lee et al., 2010b). As a member of CREB3 subfamily
of bZIP proteins, CREBH is mainly expressed in the liver,
and its expression and activity is enhanced under fasting or
by insulin resistance. Thus, CREBH and CRTC2 can be also
activated at the same time to enhance a new set of target genes.
Interestingly, binding sites of CREBH were identified among
5’-upstream sequences of gluconeogenic genes PEPCK and
G6Pase, which are distinct from the well-characterized CRE
sequences for CREB binding. Depletion of CRTC2 in the liver
almost completely abrogated CREBH-dependent activation of
gluconeogenesis in mice, showing the importance of CREBH
in the control of CRTC2-dependent glucose metabolism in the
liver. Based on these results, CRTC2 can control hepatic glucose
metabolism by interaction with various bZIP transcription factors
such as CREB, ATF6, and CREBH. Further study is required
to delineate potential interaction of other bZIP factors with
CRTC2 in the control of glucose metabolism or other metabolic
pathways in the liver.
(SHP), also known as NR0B2, is a member of nuclear hormone
receptors that lacks DNA binding domain, and inhibits activity of
other transcriptional factors by protein-protein interaction. SHP
is highly expressed in the liver, and its expression is induced by
metformin, an anti-diabetic drug that also enhances activity of
AMPK (Kim et al., 2008). By interaction with CREB, SHP disrupts
CREB/CRTC2 interaction, thus reducing hepatic gluconeogenesis
in mice (Lee et al., 2010a). Recently, transcription factor 7-like
2 (TCF7L2), a transcriptional regulator of Wnt/beta catenin
signaling pathway, was shown to control hepatic glucose
metabolism. Expression of TCF7L2 is increased under feeding
conditions in the liver, resulting in the increased occupancy of
this protein unto the cognate binding sites that are adjacent to
the CREB binding sites present in the gluconeogenic promoters
(Oh et al., 2012). Thus, CREB/CRTC2 binding to the promoter is
effectively inhibited under feeding conditions, providing another
layer of regulation in controlling glucose homeostasis in the liver.
Interestingly, TCF7L2 expression is reduced by insulin resistance,
showing reduced TCF7L2 occupancy over gluconeogenic
promoters that could also contribute to the increased expression
of gluconeogenic genes by CREB-CRTC2 transcriptional
machinery in this setting. These results suggest the involvement
of various signaling pathways in the control of CRTC2 activity.
Functions of other CRTC proteins and
roles of CRTC orthologues in other
organisms
Expression of CRTC1 is highly restricted in the central nervous
system. In particular, CRTC1 protein is found to be most highly
expressed in the hypothalamic arcuate cells. Upon feeding,
A couple of reports identified inhibitory proteins that interfere
dephosphorylation and activation of CRTC1 is promoted in the
with CRTC2 action in the liver. Small heterodimer partner
hypothalamus. Activated CRTC1 then functions as a CREB
coactivator and controls transcription of
cocaine- and amphetamine-regulated
transcript (CART), an anorexigenic protein
at the level of transcription, thus involved
in the control of feeding behavior. Indeed,
systemic knockout of CRTC1 in mice
resulted in the obesity phenotype that is
associated with leptin resistance, showing
the importance of this protein in the
control of CNS-directed control of feeding
behavior and obesity (Altarejos et al.,
2008). On the other hand, CRTC3 is shown
to be highly expressed in white and brown
adipocytes. Normally, catecholamine
induces lipolysis in white adipocytes
and fatty acid oxidation and uncoupling
protein-mediated heat dissipation in brown
adipocytes, via a binding of beta adnergic
receptors and subsequent activation of
Figure 2 i Regulation of hepatic glucose metabolism by CRTC2. A model showing
G
protein-coupled adenylate cyclase
transcriptional regulation of hepatic glucose metabolism by CRTC2. See the main text for a detailed
activity and cAMP signaling pathway.
explanation.
Inhibitory proteins that are involved in
the control of CRTC2 activity
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Role of CRTC2 in the control of hepatic gluconeogenesis
Interestingly, CRTC3 knockout mice were protected against the
diet-induced obesity, in part by enhanced energy expenditure,
and also displayed improved insulin sensitivity. It turned out that
one of the major transcriptional targets of CRTC3 is regulator of
G protein signaling 2 (RGS2), which encodes GTPase-activating
protein that limits G protein activity (Song et al., 2010). Thus,
CRTC3 knockout mice displayed prolonged activation of cAMPdependent pathway in adipocytes that would increase lipolysis
(white adipocytes) and energy expenditure (brown adipocytes),
protecting the mice from diet-induced obesity and insulin
resistance.
CRTC was also shown to be involved in the control of energy
metabolism in the lower eukaryotes. In Drosophila Melanogaster,
only a single orthologue of CRTC, termed dTORC, is present.
Deletion of dTORC in the fly resulted in the reduced lipid
accumulation and enhanced sensitivity to fasting. dTORC is
mainly expressed in the brain, and controls energy balance and
resistance to the oxidative stress, resulting in the prolonged
longevity (Wang et al., 2008). As in the case of mammalian
orthologue, dTORC activity is regulated by phosphorylation of
key serine residue. In the fly, SIK, a member of AMPK related
kinase, is involved in the phosphorylation of dTORC, and
its activity is induced under feeding conditions. Ultimately,
phosphorylated dTORC undergoes proteasomal degradation. In
Caenorhabditis elegans, also only a single orthologue of CRTC,
termed CRTC-1, is identified to date. As in the case of the
mammalian orthologue, its activity is controlled by AMPK- and
calcineurin-dependent phosphorylation events. In this organism,
CRTC-1 functions as a coactivator of CRH-1, an orthologue of
CREB. While dTORC promotes resistance to oxidative stress and
increased longevity in the fly, CRTC-1 appears to impair longevity
in C. elegans, since the deletion of either CRTC-1 or CRH-1
increases the longevity of this animal (Mair et al., 2011). Further
study will be necessary whether dTORC and CRTC-1 regulate
expression of different subset of genes to control the oxidative
stress or aging pathways in the respective organism (See table 1
for comparison).
Concluding remarks
In this review, we delineate the structure, function, and the
regulatory mechanisms of CRTC2 as a master regulator of
hepatic glucose metabolism (Figure 2). CRTC2 is activated
under fasting conditions, via inhibition of AMPK and its related
kinases, activation of PP4/calcineurin phosphatases, PRMTs,
and CBP/p300. Activated CRTC2 binds to CREB or CREBH
onto the gluconeogenic promoters in the nucleus, enhancing
gluconeogenesis at the level of transcription. Conversely, feeding
can effectively turn off transcription of gluconeogenic genes
by inhibition of CRTC2 via activation of kinases, inactivation of
phosphatases, and activation of inhibitory proteins SHP and
TCF7L2. In addition, CRTC2 is shown to transcriptionally regulate
various target genes including PGC-1α, ERRγ, and Lipin1 to
control energy metabolism and insulin signaling pathway in
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© 2014 Bio Design
the liver. Further study involving the identification of additional
transcription factors that function as partners for CRTC2 could
elicit additional CRTC2-controlled signaling pathway in the
future.
ACKNOWLEDGEMENTS
This work was supported by the National Research Foundation of Korea
(grant nos.: NRF-2010-0015098 and NRF-2012M3A9B6055345), funded
by the Ministry of Science, ICT & Future Planning, Republic of Korea,
and a grant of the Korean Health technology R&D Project (grant no :
HI13C1886), Ministry of Health & Welfare, Republic of Korea, and a grant
from Korea University.
AUTHOR INFORMATION The authors declare no potential conflicts of interest.
Original Submission: Nov 30, 2014
Revised Version Received: Dec 9, 2014
Accepted: Dec 12, 2014
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